Wellbores are typically drilled using a drilling string with a drill bit secured to the lower free end and then completed by positioning a casing string within the wellbore and cementing the casing string in position. The casing increases the integrity of the wellbore and provides a flow path between the surface and selected subterranean formation for the injection of treating chemicals into the surrounding formation to stimulate production, for receiving the flow of hydrocarbons from the formation, and for permitting the introduction of fluids for reservoir management or disposal purposes.
Perforating has conventionally been performed by means of lowering a perforating gun on a carrier down inside the casing string. Once a desired depth is reached across the formation of interest and the gun is secured, it is fired. The gun may have one or many charges thereon which are detonated using a firing control, which is activated from the surface via wireline or by hydraulic or mechanical means. Once activated, the charge is detonated to penetrate (perforate) the casing, the cement, and to a short distance, the formation. This establishes the desired fluid communication between the inside of the casing and the formation. After firing, the gun is either raised and removed from the wellbore, left in place, or dropped to the bottom thereof.
Perforating guns used in service operations for perforating a formation typically include an elongated tubular outer housing within which is received an elongated tubular carrier, which carries a number of shaped charges. The tubular carrier is located relative to the housing to align shaped charges with reduced-thickness sections of the outer housing. Various means are well known in the art to retain each charge in place, including twist locks, snap rings, tabs and o-rings.
Perforating guns often require a support sleeve to increase the outer housing collapsible-pressure rating, the pressure at which the outer housing will catastrophically deform due to pressure in the wellbore, so that the perforating gun may operate. It is well known in the art that the collapsible-pressure rating can be increased by positioning a support sleeve within the outer housing that is sized to transfer and distribute surrounding pressure from the outer housing to the support sleeve. Because the tubular carrier must be positioned within the support sleeve, typically composed of heavy steel, the shape and size of the charge that can be used is limited. Another disadvantage is that the support sleeve increases the distance between the charge and the formation. As a result, perforation results and/or the collapsible-pressure rating may be compromised. Another disadvantage of the support sleeve is the necessity for zinc charge holders. While zinc is more effectively pulverized than steel and may be dissolved with acid treatment, it is less desirable than steel charge holders that yield better perforating results.
A need, therefore, exists for an apparatus which improves perforation results and the collapsible-pressure rating for a perforating gun. A further need exists for an apparatus which permits the use of steel charge holders.